The vast majority of TPU fibers are made by a dry spinning process involving dissolving the TPU in solvent. The melt spinning process to produce TPU fibers has several inherent advantages including less cost, less capital cost, and less environmental concerns as a result of not using volatile solvents. Dry spun fibers are made from a TPU having urea linkages. Therefore, the dry spun fibers are heat resistant, but this type of TPU cannot be melt spun as it will not melt. Also, typical spandex TPU fibers without modification are attacked by bleach and suffer from a loss of tenacity and power when exposed to bleach or chlorinated swimming pools. Typical unmodified spandex has low dye pickup with dispersive dyes and has poor wet fastness.
TPU compositions for making melt spun fibers contain a TPU polymer made from a hydroxyl terminated intermediate which is typically a polyether or polyester end capped with a hydroxyl group; a polyisocyanate, such as a diisocyanate; and a hydroxyl terminated chain extender. The hydroxyl terminated intermediate forms the soft segment of the TPU polymer while the polyisocyanate and the chain extender forms the hard segment of the TPU polymer. The combination of soft and hard segments give the TPU polymer elastic properties. The TPU polymer is also frequently lightly cross linked by using a pre-polymer end capped with a polyisocyanate. The cross linking material is added to the melted TPU polymer during spinning.
Polyester and polyether based TPU have both been used in melt spinning TPU fibers. Polyether based TPU fibers can be made to have better hydrolytic stability than TPU fibers made from polyester based TPU.
Polyether TPU fibers have better properties if the polyether hydroxyl terminated intermediate used to make the TPU is higher in molecular weight. Unfortunately, polyether TPU's using a higher molecular weight hydroxyl terminated intermediate tend to have a shorter run time before experiencing pressure build up in the spinneret, thus leading to fiber breakage. When fibers break in a spinning operation, the production line must be shut down and the spinneret cleaned. This is a costly down time and production is lost.
It would be desirable to be able to use TPU polymers which are made from a polyether intermediate having higher molecular weight while retaining the long run time of TPU made with a low molecular weight polyether intermediate.
It would also be desirable to have melt spun TPU fibers which have good resistance to bleach, good dye pickup, color fastness and able to withstand the dyeing conditions of polyester fibers.